The mammalian thyroid gland contains two different parenchymal cell types. One type, follicular cells are of endodermal origin and produce the hormones thyroxine and tri-iodothyronine. The second type, parafollicular cells are of neural crest origin and produce the peptide hormones, calcitonin and somatostatin as well as the biogenic amine, serotonin. The two types of parenchymal cells are contained within the basal laminae of thyroid follicles and are connected by desmosomes. We propose, as a working hypothesis, that this close spatial relationship between follicular and parafollicular cells is functionally meaningful and evolved to subserve the role of facilitating parafollicular to follicular cell communication. We further propose that the biogenic amine, serotonin, is secreted by parafollicular cells and serves as the messenger of intercellular communication. A corrollary of the overall hypothesis is that the parafollicular cells retain neuron-like properties, resemble serotonergic neurons, and function as para-neurons. Several specific aims will be tested in the proposed research. (1) What is the mechanism of serotonin storage in parafollicular cells? This will be determined by determining whether serotonin, somatostatin, calcitonin and serotonin binding protein (SBP) are all stored together in the same subcellular granules or vesicles of parafollicular cells using cell separation and fractionation methods. (2) How does serotonin accumulate in parafollicular cell granules? This will be studied (i) by examining the role of the protonmotive force hypothesis and serotonin uptake and (ii) by determining if serotonin exists as a complex with SBP within parafollicular cells under physiological conditions in vivo. (3) We will also test to determine if Ca++, the natural parafollicular cell secretogogue, or TSH, release SBP along with serotonin and calcitonin. Moreover, we will like to determine if a portion of SBP is retained on the plasma membrane of secreting parafollicular cells using electron microscope immunocytochemical methods and do secretory granule recycle. (4) We also propose to study where are the serotonin receptors on follicular cells that are responsible for serotonin's activation of these cells using radioligands and pharmacological manipulative procedures. We will also test to determine if parafollicular cell secretion produces a response in adjoining follicular cells and is this response mediated by serotonin. Knowledge of basic secretory mechanisms of parafollicular cell activity will help to enhance understanding the role of this rare cell type in thyroid physiology.